1. Field of the Invention
The present invention relates to a temperature controller for detecting the temperature of an object to be heated, and controlling a drive operation of a heating member for heating the object to be heated according to the detected temperature.
2. Description of the Related Art
In an electrophotographic recording apparatus such as a laser printer, a heat roll is provided in a fixing device for fixing, on a paper sheet, a toner image transferred from a developer thereto. The heat roll is heated by a heater and melts a toner attached to a paper sheet. When the paper sheet with the melt toner is clamped between the heat roll and a pressure roller, the melt toner is fixed onto the paper sheet under pressure.
In a heat roll of this type, the temperature of the roll must be kept constant so as to obtain a stable image and to assure safety. For this purpose, the temperature control of the heat roll is performed, and is often realized by thermal feedback control.
According to a conventional heat fixing device, a heater arranged in a heat roll is heated by energization from a power supply through a switch and a temperature over-rise prevention device such as a fuse or a thermostat. In this case, in a temperature controller used in the heat fixing device, a temperature sensor detects the temperature of the heat roll, and supplies a voltage according to the detected temperature to a comparator. The comparator compares the voltage corresponding to the detected temperature with a reference voltage corresponding to a target temperature, thereby ON/OFF-controlling the switch.
With this control system, when the output voltage from the temperature sensor is smaller than the reference voltage, i.e., when the temperature of the heat roll is lower than the target temperature, the switch is turned on, and the heater is heated. In contrast to this, when the output voltage from the temperature sensor is larger than the reference voltage, i.e., when the temperature of the heat roll is higher than the target temperature, the switch is turned off, and heat generation of the heater is stopped.
In the temperature controller with the above-mentioned arrangement, when the temperature of the heat roll is held at the target temperature, temperature change is very small, and temperature control can be satisfactorily performed. In an initial heating state from an inactive state, the temperature sensor cannot follow the temperature rise of the heat roll, and the temperature detected by the sensor becomes undesirably lower than the actual temperature.
For this reason, when the detected temperature reaches the target temperature, the actual temperature of the heat roll has already exceeded the target temperature. In this manner, when the temperature of the heat roll exceeds the target temperature, the service life of the heat roll or the heater may be shortened, and the temperature over-rise prevention device may be erroneously operated.